The present invention relates to an ion source, a mass spectrometer, an elemental analyser, a method of ionising a sample, a method of mass spectrometry and a method of elemental analysis of a sample.
A principal use of mass spectrometers is to determine the mass to charge ratio of ions generated from an unknown substance in order to provide information from which to aid the identification of the substance. Where the unknown substance comprises one or more organic compounds it is commonly necessary to determine the elemental composition of these compounds. This information is helpful, and often essential, for the identification of the organic compounds present in the unknown substance.
The measurement of the mass of an organic compound is rarely adequate information from which to determine the elemental composition of the compound. The element carbon may combine with any or all of the elements hydrogen, nitrogen, oxygen, sulphur, phosphorous, fluorine, chlorine and bromine (which are the most common constituents of organic compounds) in numerous different proportions so that it is likely that an organic compound with a given nominal molecular weight will have a large number of possible elemental compositions.
The different isotopes of the different elements do not have precise integer masses but instead have a small mass sufficiency or deficiency (of the order of +/− a few hundredths of a mass unit) with respect to its nominal or integer mass. Hence, the exact mass of an organic molecule is not an integer and is generally not precisely the same as those of other organic molecules with the same nominal or integer mass. Hence, if the measurement of the molecular weight is made to a higher accuracy it is possible to eliminate a large number of possible elemental compositions that have the same nominal or integer mass. Accurate mass determination reduces the number of possible elemental compositions, and the more accurate the mass determination the smaller the number of possible elemental compositions.
Inspection of the isotopic distribution of the molecular ion may also help to reduce the number of possible elemental compositions. For example, the presence of chlorine and/or bromine is usually easily recognised since these elements have very distinct isotope distributions. The isotope ratio for chlorine, Cl35/Cl37, is approximately 3, and the isotope ratio for bromine, Br79/Br81, is approximately 1, both of which are quite different to those of other commonly occurring elements in organic compounds. The element sulphur also has a relatively distinctive isotope ratio. The ratio S32/S34 is approximately 22.5 and with careful measurements of the isotope ratios of a molecular ion it is sometimes possible to determine if sulphur atoms are present in the molecule, and if so approximately how many. However, it is considerably more difficult to determine if sulphur is present or not if the molecules of interest also contain either or both of chlorine and bromine. Information regarding the likely number of chlorine and bromine atoms in the molecule, or in the absence of chlorine and bromine, the range within which the number of sulphur atoms are likely to be present in the molecule, will help reduce the number of possible elemental compositions for an unknown organic molecule analysed by mass spectrometry.
Apart from the determination of the likely number of chlorine and/or bromine atoms in each molecule, or, in the absence of chlorine and bromine, the determination to a lower precision of the approximate number of sulphur atoms in each molecule, it is very difficult or impossible to determine anything very useful from the molecular ion isotope distribution about the presence and relative numbers of the other common elements occurring in organic compounds. In particular, fluorine and phosphorous are mono-isotopic and hydrogen, nitrogen and oxygen have very low abundance secondary isotopes, and their presence in an organic molecule is not revealed in its molecular ion distribution.
When necessary, it is common practice to resort to other techniques in order to determine the elemental composition of an unknown organic compound. For example, elemental analysers may be used to determine the presence of certain types of element in the molecule.
There are several known methods for elemental analysis. In the more common types of elemental analyser a sample to be analyzed is weighed into a disposable tin or aluminium capsule. The sample is injected into a high temperature furnace and combusted in pure oxygen under static conditions. At the end of the combustion period, a dynamic burst of oxygen is added to ensure total combustion of all inorganic and organic substances. If tin capsules are used for the sample container, an initial exothermic reaction occurs raising the temperature of combustion to over 1800° C.
The resulting combustion products pass through specialized reagents to produce from the elemental carbon, hydrogen, and nitrogen: carbon dioxide (CO2), water (H2O), nitrogen (N2) and N oxides. These reagents also remove all other interferences including halogens, sulphur and phosphorus. The gases are then passed over copper to scrub excess oxygen, and to reduce oxides of nitrogen to elemental nitrogen.
The resulting mixture of gases may then be separated, for example by gas chromatography and/or may be analysed, for example, using specific detectors. The gas mixture may be measured using a series of high-precision thermal conductivity detectors, each containing a pair of thermal conductivity cells. A water trap is provided between the first two cells. The differential signal between the cells is proportional to the water concentration which is a function of the amount of hydrogen in the original sample. A carbon dioxide trap is provided between the next two cells for measuring carbon. Finally, nitrogen is measured against a helium reference.
Sulphur is commonly measured separately, as sulphur dioxide, by replacing the combustion and reduction reagents. Oxygen is also commonly measured separately by pyrolysis in the presence of platinized carbon. The oxygen is finally measured as carbon dioxide.
Known elemental analysers are not particularly sensitive when compared with that commonly achieved by mass spectrometry. Typically 1-5 mg of sample is required, or more for samples with low carbon content. The analysis time is quite long, typically of the order of 5 minutes for carbon, hydrogen and nitrogen analysis. Hence, the technique is too slow to be used when directly coupled to gas or liquid chromatography. Hence the sample needs to be purified separately before analysis. Furthermore, if the sample is in solution, as would be the case for separation by liquid chromatography, it is necessary to isolate, collect and desolvate a sample before submission for elemental analysis.
More recently the technique of gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS) has been developed wherein a mixture of organic materials are separated by gas chromatography, combusted to carbon dioxide, water and other oxides, and the 13C/12C isotope ratio is then measured by isotope ratio mass spectrometry. The effluent from a gas chromatography capillary column is arranged to first enter a motorised valve to allow solvent to be diverted to waste to prevent premature depletion of the combustion reactor. The analytes eluting from the capillary column are then directed to an alumina or quartz combustion tube loaded with an oxidising reagent such as copper oxide (CuO), nickel oxide (NiO) or zinc oxide (ZnO). A mixture of oxidising reagents may be used and a catalytic material may also be added. For example, the combustion tube may be loaded with a twisted strand of copper, platinum and nickel wires. The combustion tube is heated, typically to between 900° C. and 950° C., and is periodically recharged with oxygen to convert the surface layers of the copper wire to copper oxide and the nickel wire to nickel oxide. Following combustion, the effluent is dried in a Nafion® water trap or cryogenic water trap and the dried effluent is admitted to the isotope ratio mass spectrometer.
In comparison to elemental analysis isotope ratio mass spectrometry (EA-IRMS), the GCC-IRMS method requires lower levels of analyte i.e. nanomoles versus micromoles of carbon. It is also faster with the gas-phase combustion process occurring on a millisecond time scale. GCC-IRMS has proven to be adequate for fast GC detection thereby providing the convenience of on-line isolation of components. However, this method is not appropriate for the measurement of the isotope ratios of all the elements commonly occurring in organic compounds and for similar reasons it is not appropriate for the elemental analysis of organic compounds.
In summary, accurate mass measurement of an organic compound using mass spectrometry is, in itself, rarely adequate for determining the elemental composition of the organic compound. The determination of the elemental composition is aided by the information gained from separate measurements, such as the information gained from measurements with an elemental analyser. However, elemental analysers are relatively insensitive and are relatively slow. If the sample is a mixture of organic compounds, as the vast majority of samples are, then it is necessary to isolate the components of the mixture prior to the elemental analysis. The speed of the elemental analyser does not allow on-line interfacing to chromatography. Furthermore, liquid chromatography would require the additional process of removing solvent before submission of the effluent material to the elemental analyser. The methods employed in GCC-IRMS allow on-line interfacing to gas chromatography, and provide improved sensitivity, but are not suitable for elemental analysis. The methods employed in GCC-IRMS are also not appropriate to liquid chromatography since it is necessary to remove all solvent material before submission of the effluent material to the combustion chamber. Finally, there is no method for the elemental analysis of ionised organic molecules, or for the elemental analysis of fragment ions, daughter ions, or decomposition or reaction product ions of organic molecules.
It is desired to provided an improved apparatus and method for determining the elemental composition of an organic compound and/or for determining an isotope ratio of one or more elements present in an organic compound.